La Paz and its surrounding regions range from about 3200m to about 4400m above sea level. (Nearby Illimani, which was shrouded in clouds the whole time I was there, reaches 6,438m). For those of us who dwell in valleys near the ocean, there is no particularly good way to arrive at such a place. I hiked up California's Mount Lassen a couple of years ago and despite about three miles of huffing and puffing, I saw only an altitude that would probably not get me into La Paz at all. A ballooning bag of potato chips went only as far up as the base of Lassen where the parking lot is. Water boils at 87C at these altitudes.
I spent a few days before my trip in the mountains in northern California, a higher altitude than home but a far lower altitude than my destination. I do not know whether it really helped very much. Despite flying into El Alto, I did not have any trouble with altitude sickness as such. At least, I didn't have a headache or feel queasy.
I did feel very out of breath for the first few days, any time I tried to exert myself. Many of the streets are quite steep. Some sidewalks and many alleys consist of stairs, and my friend's house is up a hill and then some stairs, meaning that it is a long, steep climb from the nearest place the taxi could leave us.
The first few days, I accepted help carrying luggage. I packed as lightly as I could, but I packed books and gifts for my friend. Walking slowly wasn't too difficult, but climbing anything meant stopping every flight or two of stairs to catch my breath, and even catching my breath took longer.
People who live in La Paz and elsewhere in the Andes and the Altiplano develop larger hearts and lungs to compensate, but I think even people who live there all the time still tire sometimes.
By the end of three weeks there, I was making it up the hill to my friend's house all in one go. I even tried racing him up it once. He won, but I think I put in a pretty good attempt, for a foreigner and flatlander.